This invention relates generally to radio receivers for microwaves and SHF waves, and particularly to preamplifiers for such radio receivers.
With recent advance in noise reduction in connection with GaAs FETs, most advanced GaAs FETs show a noise figure which is less than 1.5 dB at 12 GHz. Following with this noise reduction, GaAs FETs are sometimes used as an amplifying element constituting a preamplifier of a radio receiver of SHF band. However, there have been various problems to be solved in order that GaAs FETs are used as an amplifying element of such a preamplifier. One of the most difficult problems is that the input impedance of a GaAs FET deviates from an impedance matching state when the noise figure of the GaAs FET is minimum, whereas the noise figure shows a minimum value when VSWR is around 3. Using a preamplifier showing large input VSWR for an SHF band receiver, results in a problem of matching between the SHF band receiver and an antenna conneted thereto.
For this reason therefore, measures have hitherto been taken for obtaining desired matching with respect to an antenna by either (1) adding an isolator having a low insertion loss at an input stage of a preamplifier designed to show minimum noise figure or (2) designing the input impedance of a preamplifier to show impedance matching for maximum gain.
However, the above method (1) suffers from a problem that the SHF band receiver would be complex in structure and high in cost because an isolator of extremely low insertion loss is needed. Furthermore, the method (1) suffers from a drawback that the noise figure of the SHF band receiver deteriorates as much as the insertion loss of the isolator. In the method (2), since the preamplifier is designed to show impedance matching for maximum gain, the noise figure of the amplifier greatly deteriorates when compared to a case of impedance matching for minimum noise. Furthermore, the method (2) has a drawback that an input matching circuit would be complex in structure, and the noise figure of the preamplifier would be deteriorated greatly due to the increase in loss of the input signal at the input matching circuit.